The invention relates generally to the field of optical measurements and measuring systems, and more particularly to a method and system for optical spectrum analysis that utilizes optical heterodyne detection.
Dense wavelength division multiplexing (DWDM) requires optical spectrum analyzers (OSAs) that have higher spectral resolution than is typically available with current OSAs. For example, grating-based OSAs and autocorrelation-based OSAs encounter mechanical constraints, such as constraints on beam size and the scanning of optical path lengths, which limit the spectral resolution that can be obtained.
As an alternative to grating-based and autocorrelation-based OSAs, optical heterodyne detection systems can be utilized to monitor DWDM systems. For example, optical heterodyne detection systems can be utilized for optical spectrum analysis of an input optical signal. FIG. 1 is a depiction of a prior art heterodyne-based OSA that includes an optical coupler 110 that combines an input signal 102 from an input fiber 104 with a swept local oscillator signal 106 from a local oscillator source 105 via local oscillator fiber 108. The combined optical signal travels on an output fiber 118 and is detected by a heterodyne receiver 112. The heterodyne receiver converts optical radiation from the combined optical signal into an electrical signal. Square law detection results in mixing of the two combined optical signals and produces a heterodyne beat signal at a frequency that is equal to the frequency difference between the combined optical signals. The heterodyne beat signal is processed by a signal processor 116 to determine a characteristic of the input signal, such as frequency, wavelength, or amplitude.
In an ideal heterodyne-based OSA, the local oscillator source produces a local oscillator signal that sweeps over a range of optical frequencies (or wavelengths) at a constant rate (i.e., dxcexd/dt=constant, where xcexd is the optical frequency). Although a constant sweep rate is ideal, known local oscillator sources produce local oscillator signals that sweep at non-uniform rates. FIG. 2 depicts an example graph of a local oscillator signal 206 that has a non-uniform sweep rate. The non-uniformity of the local oscillator signal sweep rate causes inaccurate frequency measurements of the input signal. FIG. 3 depicts an example input signal spectrum, including multiple DWDM channels 309, which may be input into the heterodyne-based OSA. As depicted in FIG. 3, each of the DWDM channels is separated by a frequency band of the same width. FIG. 4 depicts an example output from a heterodyne-based OSA that results from the non-uniformly swept local oscillator signal depicted in FIG. 2 in relation to the input signal spectrum of FIG. 3. As shown in FIG. 4, the measured spectrum that results from the non-uniformly swept local oscillator signal does not accurately reflect the actual input signal spectrum. Specifically, the non-uniformity in the local oscillator signal sweep rate translates directly into errors in the accuracy of the measured signal spectrum (as indicated by the channels that have a different channel spacing than the channels depicted in FIG. 3).
In view of the need for higher resolution OSAs and the problems caused by the non-uniform sweep rate of local oscillator sources used in heterodyne-based OSAs, what is needed is a heterodyne-based OSA that can correct for the non-uniformities in the sweep rate of a local oscillator source.
A method and system for heterodyne-based optical spectrum analysis involves measuring the sweep rate of the swept local oscillator signal and then generating an output signal that accounts for non-uniformities in the sweep rate of the swept local oscillator signal. In an embodiment, an input signal is combined with a swept local oscillator signal in an optical coupler and the sweep rate of the swept local oscillator signal is measured in a relative frequency measurement system. The combined optical signal is output from the optical coupler to a receiver and a heterodyne beat signal is generated. The heterodyne beat signal and measured local oscillator frequency sweep rate information are utilized by a signal processor to generate an output signal that is indicative of an optical parameter of the input signal and that accounts for non-uniformities in the sweep rate of the local oscillator signal. Because the actual sweep rate of the swept local oscillator signal is measured during analysis of the input signal, the optical frequency scale accuracy of heterodyne-based OSAs is improved.
A system for optical spectrum analysis includes a local oscillator source, an optical coupler, a heterodyne receiver, a relative frequency measurement system, and a signal processor. The local oscillator source generates a swept local oscillator signal that sweeps across a range of frequencies. The optical coupler has a first input, a second input, and an output. The first input being optically connected to receive an input signal, the second input being optically connected to the local oscillator source to receive the swept local oscillator signal, and the output being optically connected to output a combined optical signal that includes the input signal and the swept local oscillator signal. The heterodyne receiver has an input for receiving the combined optical signal from the optical coupler and an output for outputting a heterodyne beat signal that is representative of the combined optical signal. The relative frequency measurement system is optically connected to the local oscillator source and generates measured local oscillator frequency sweep rate information in response to the swept local oscillator signal. The signal processor receives the heterodyne beat signal from the optical receiver and the measured local oscillator frequency sweep rate information from the relative frequency measurement system and generates an output signal that is indicative of an optical parameter of the input signal.
A method for optical spectrum analysis that utilizes optical heterodyne detection involves providing an input signal, providing a swept local oscillator signal that sweeps across a range of frequencies, measuring the frequency sweep rate of the swept local oscillator signal to generate measured local oscillator frequency sweep rate information, combining the input signal with the swept local oscillator signal to create a combined optical signal, detecting the combined optical signal to generate a heterodyne beat signal, and generating an output signal, from the filtered heterodyne beat signal and the measured local oscillator frequency sweep rate information, that is indicative of an optical parameter of the input signal.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.